The present invention relates to a current detecting device by which an overcurrent can be detected with an operational amplifier of differential amplification type, wherein the device has a temperature compensating function for the current detection value or current limit value.
FIG. 7 shows a circuit diagram for a current limiting device together with a switching element to be protected, by which a switching element is protected from being destroyed by detecting an overcurrent flowing through the switching element. A current limiting device 30 controls a current flowing from a power supply 1 to a load 2, and is equipped with an output stage MOSFET 7 to control a main current 4 by a driving circuit 5, a sensing MOSFET 6 by which a shunted current 3 shunted from the main current 4 is controlled, and a current detecting device 25 by which the value of the shunted current 3 flowing to the sensing MOSFET 6 is detected.
The current detecting device 25 has a sensing resistance 11 which generates a voltage drop corresponding to the value of the shunted current 3, a voltage source circuit 10 which generates a reference voltage to compare with the detected voltage drop at the sensing resistance 11, and a comparator by which the reference voltage and the detected voltage are compared. In this device, an operational amplifier 9 of differential amplification type is used as the comparator.
Further, the output of this operational amplifier 9 is supplied to a logic circuit 8. The logic circuit 8 judges if a value of the shunted current 3, i.e. a value of the main current 4, is in a state of an overcurrent based on an output value of the operational amplifier 9. In case of an overcurrent, the logic circuit 8 outputs a signal for the drive circuit 5, so that the main current 4 flowing in the output stage MOSFET 7 is limited.
In such current limiting device 30 or current detection device 25, one of problems is the fluctuations of resistance value of the sensing resistance 11 according to temperature change. For instance, when resistance of the sensing resistance 11 has a positive temperature dependency, an overcurrent state may not be judged even if the main current is in an overcurrent condition at a low temperature. On the other hand, at a high temperature it is judged that an overcurrent state occurs even if the main current 4 is less than the range of the overcurrent.
Then, in the conventional device 30 or device 25, a current-mirror circuit as shown in FIG. 8 is installed in the operational amplifier 9, and input resistances 17 of the current-mirror circuit are adjusted to become an appropriate temperature dependency. Here, the input resistances 17 are inserted between a power supply 21 and each of a (+) side input terminal 18 and a (-) side input terminal 19. Numeral 20 is a constant current supply source.
The input resistances 17 are formed of well-known semiconductor resistances. Since a temperature dependency rate of a resistance value of a semiconductor changes according to the composition of the semiconductor, the semiconductors made of an appropriate composition are chosen, so that the temperature dependency of an offset voltage of the operational amplifier 9 is determined.
For instance, a temperature dependency rate KR (a ratio of the change of a semiconductor resistance value relative to the change of temperature Tj) in a semiconductor resistance composed of the P+ layer of the polysilicon becomes about 1/2 of the temperature dependence rate KR of a semiconductor resistance composed of the P+layer of silicon (Si), as shown in FIG. 9. Therefore, the temperature dependency of an offset voltage of the operational amplifier 9 can be adjusted by using semiconductor resistances for the input resistances 17 to thereby compensate the temperature dependency of the sensing resistance 11. Therefore, the overcurrent detecting device with high accuracy and the current limiting device can be made.
However, in order to form an overcurrent detecting device with high accuracy or a current limiting device where the temperature dependency is compensated by using such sensing resistances 11, it is required to install the semiconductor resistances having different temperature characteristics respectively, i.e. semiconductor resistances of different compositions. Since different manufacturing processes are necessary to form the different semiconductor resistances, in the conventional method, the process requires many steps and is complex when comparing to the case of manufacturing a device without the compensating function. Thus, the manufacturing term and cost increase.
In view of the above problems, in the present invention, the compensating function to the temperature dependency of a sensing resistance is formed without using semiconductor resistance of different temperature characteristics, and it is an object of the invention to provide a current detecting device which can be manufactured in a short term and low cost.